1. Field of the Invention
The present invention relates to a servo system in optical information recording and/or reproducing apparatus (the term "information recording and/or reproducing apparatus" used herein includes all apparatus that perform at least one of recording and reproduction of information), and more particularly, to the optical information recording and/or reproducing apparatus having a stabilized loop for reducing the effect of high-frequency secondary resonance of an actuator.
2. Related Background Art
In the optical information recording and/or reproducing apparatus that are conventionally available, that is, in the information recording and/or reproducing apparatus of an optical disk, a light beam for recording or reproducing information irradiates the optical disk in the form of a fine spot through an objective lens. An information track or information tracks are formed in a spiral shape or in a concentric shape on the optical disk, and information is sequentially read or written during rotating of the optical disk. The rotation of the optical disk causes surface deviation of the optical disk at a point irradiated by the light spot, and the deviation between the rotational axis and the information track causes track deflection. At this time focus control is carried out to move the objective lens along the optical axis in order to focus the fine spot on the disk information surface. Further, tracking control is also carried out to move the objective lens in the track-traversing direction so as to keep the fine spot continuously to follow up the information track.
FIG. 1 shows a block diagram of this focus servo system and tracking servo system. In FIG. 1, reference numeral 1 designates a target value of the servo system, which is a target position in the focusing direction of the disk information surface in the case of the focusing servo or a target position in the tracking direction of the information track in the case of the tracking servo. Numeral 2 represents a feedback point, from which the difference between the target value 1 and an actual position of the light spot is output as an error signal. In a practical apparatus, this error signal is obtained from an output from a servo sensor, and, in this case, it is a negative feedback point comprising a negative feedback circuit. Numeral 3 is an open-loop gain, where an open-loop transfer gain of the servo loop is collectively set. Numeral 4 denotes a phase compensation block, in which compensation is effected in order to stabilize the servo loop. Specifically, it is the phase compensation block for phase lag compensation in the low frequency region, phase lead compensation in the high frequency region, etc. A torque constant represented by numeral 5 and an actuator represented by numeral 6 comprise a drive torque of the actuator for actually moving the light spot in the focusing or tracking direction, and the actuator.
When a loop of the focusing servo system is set in this manner, the loop operates to stabilize the servo system from the sensor to the focusing actuator so that the characteristic S-shaped curve, for example produced from the difference between cross sensor elements in a quarterly divided focusing sensor, may become zero, whereby the position of the objective in the optical-axis direction is always maintained at the optimum spot point. Similarly, when a loop is set by switching the tracking servo system on, the operation for always continuing following up a predetermined track is carried out by forming a negative feedback loop to the tracking actuator in accordance with an output from the tracking sensor.
Further, the optical disk apparatus has a seek function to greatly move the light spot in the radial direction of the disk in order to permit random access to information. Normally, this function, called a seek for jumping to a track at a remote point, is realized by control for moving the objective lens or the optical head at a high speed by a voice coil type linear motor.
A block diagram of this seek velocity control system is shown in FIG. 2. In the drawing, numeral 35 denotes a target track position, which is an input of data indicating a position of an information track desired to be accessed. Numeral 36 is a residual error calculating block, which subtracts the track position where the light spot is located at present from the target track position 35 and then outputs a remaining distance for access. Numeral 34 represents a target velocity generation block, which outputs a target value for the moving velocity of the light spot according to the remaining distance. Generally, the linear motor for seeking is driven at the drive velocity in a drive profile formed like a parabolic drive, a trapezoid drive, or a triangular drive. Normally, the target velocity follows a profile in which the velocity decreases as the optical head approaches a designated track. The target velocity value 21 generated by the target velocity generation block 34 is input into a feedback point 22. The velocity of the light spot, that is, the velocity of the actuator or linear motor 26 is negatively fed back to the feedback point 22. Thus, an output from the feedback point 22 is a velocity deviation between the target velocity and the actual moving velocity of the light spot.
Further, numeral 23 stands for an open-loop gain of the negative feedback loop from the actuator velocity, which is a block for determining the open-loop gain of the velocity servo loop. Numeral 25 is a torque constant of the actuator or a linear motor, which indicates the sensitivity of the actuator. Numeral 26 designates the actuator or linear motor for moving the light spot, which is a tracking actuator in the case of the apparatus of a single actuator and in the track jump operation during seeking to a near distance. It is a linear motor in the case of the apparatus having a linear motor for moving the optical head.
In the above arrangement, the track jump is carried out as follows: the position of the actuator is detected according to the operation of the actuator or linear motor 26; the residual distance calculating block 36 calculates the difference between the position of the actuator and a component of the target track position; the target velocity generation circuit 34 obtains a target velocity value of the actuator 26 according to the difference component; the feedback point block 22 obtains the difference component between the target velocity value and the current moving velocity 32 of the actuator 26; the difference component is amplified with the gain designated by the open-loop gain block 23 of the negative feedback loop to be converted into the torque constant 25; and the actuator 26 is driven by a torque corresponding to the torque constant thus obtained. In this manner, the track jump of the optical pickup head is carried out to the target track, whereby the optical head reaches the target track through a process of rough seeking and subsequent fine seeking in the case of a two-step seek operation or in the same manner as the track jump mode in the case of a onestep seek operation, and then transferring the device into a tracking servo state.
The above conventional examples, however, exhibit various barriers preventing them from meeting recent demands to improve the performance of optical disk apparatus. In order to increase the information capacity of an optical disk, the information area can be decreased by forming a finer spot using a laser light source of a shorter wavelength. In this case, the pitch of information tracks can be decreased, but the accuracy of tracking control must be enhanced with a decrease of the pitch. Since the depth of focus of the spot becomes shallower, the accuracy of similar focus control must be enhanced.
With an improvement in performance of computers, an increased quantity of data are handed, and enormous volumes of data can be processed within a short time. Under such circumstances, data processing amounts, that is, the transfer rate of the optical disk is also desired to be improved. One way for raising the transfer rate is to increase the rpm of the disk. An increase of the rpm of the disk increases the surface deviation and the acceleration of track deflection, which results in the need to enhance the ability of focusing control and tracking control.
Similarly, the ability to seek control needs to be enhanced in order to raise the access velocity to information.
In conventional apparatus, the control ability of focusing servo was limited by the occurrence of high-frequency secondary resonance of the focusing actuator due to deflection, chattering, or disturbance resulting from low rigidity of the actuator system. FIG. 3 shows Bode diagrams of open-loop transfer characteristics of the closed loop of the conventional focusing control. As shown in the diagrams, the primary resonance frequency appears near 30 Hz and the secondary resonance frequency near 10 kHz, and the phase characteristics show rapid changes at the respective points. Here, if the secondary resonance appears at 10 kHz, as illustrated, because of the structure of the actuator, the loop becomes unstable even if the focus control band is about 3 kHz. Namely, in the case of the focusing servo system as in this example, the control band needs to be set below 3 kHz.
A method for decreasing the negative effect of the secondary resonance is to use a band-attenuation filter, called a notch filter, in the servo loop. The method using the notch filter, however, needs to employ a large Q value of the notch filter in order to decrease the phase shift around the control band so as to secure the stability of the loop. This narrows the frequency range that could be attenuated, and the frequency of the notch filter needs to be accurately coincident with the secondary resonance frequency of the actuator. Since the secondary resonance frequency and characteristics change depending upon the ambient temperature or change with age, the effect of the notch filter cannot be fully attained. Further, because the secondary resonance frequency or characteristics changes apparatus-by-apparatus, adjustment was necessary for every apparatus.
Further, the above method using the notch filter just lowered the peak of secondary resonance in the gain characteristics, but did not improve the phase characteristics. Thus, it was not able to be applied to the secondary resonance near the control band, i.e., near the zero-cross frequency.
The situation is the same as to the tracking control loop. Recent tracking actuators, for example as seen in Japanese Patent Application Laid-open No. 5-298724, tend to employ a method for performing seek and tracking over the entire surface of the disk with a single actuator. This method with a single actuator can decrease the number of actuators in the apparatus, and, therefore, can realize a low-cost apparatus as decreasing the dissipation power or decreasing the mechanical and control systems associated with drive and control. It is, however, difficult to decrease the secondary resonance or to increase the secondary resonance frequency because of the mechanism. It was thus difficult to enhance control accuracy when the actuator of this type was used.
The same problem arises as to seek control. As discussed above and shown in the block diagram of the seek velocity control system in FIG. 2, the seek operation is carried out under such control that the moving velocity or the position of the light spot is detected and the light spot moves along a target velocity or position profile. If the optical head is moved by a distance of 10 mm for the time of 20 or less ms, as seen these years, the control band during seeking needs to be set considerably high. However, the negative effect of secondary resonance of the seek actuator hindered a rise of the control band.
It is also possible to take a countermeasure using the notch filter against the secondary resonance in the case of the seek control, but the same problem as explained above as to the focus control arose in that case.